The present invention relates generally to implantable intraluminal devices, particularly intraluminal stents. Because of the open lattice found in most intraluminal stents, a primary problem with these types of devices is occlusion of the vessel occurring after stent placement. Tissue ingrowth and neointimal hyperplasia significantly reduces the open diameter of the treated lumen over time, requiring additional therapies. The present invention incorporates the use of a biocompatible barrier material that prevents or delays the tissue ingrowth and neointimal hyperplasia, thus maintaining luminal patency for longer periods after initial treatment. The use of expanded polytetrafluoroethylene (ePTFE) as a bio-inert barrier material is well documented. In accordance with certain of its preferred embodiments, the present invention utilizes a radially expandable ePTFE material, such as that described in U.S. Pat. No. 6,039,755, to partially or fully embed the stent lattice, thereby providing a suitable barrier which improves stent patency.
The inventive intraluminal stent-graft device may be implanted either by percutaneous delivery using an appropriate delivery system, a cut-down procedure in which a surgical incision is made and the intraluminal device implanted through the surgical incision, or by laparoscopic or endoscopic or endoscopic delivery. More particularly the present invention relates to shape memory alloy and self-expanding endoluminal stents which are at least partially encapsulated in a substantially monolithic expanded polytetrafluoroethylene (“ePTFE”) covering. In accordance with the present invention, an endoluminal stent, which has a reduced diametric dimension for endoluminal delivery and a larger in vivo final diametric diameter, is encapsulated in an ePTFE covering which circumferentially covers both the luminal and abluminal walls along at least a portion of the longitudinal extent of the endoluminal stent. The endoluminal stent is preferably fabricated from a shape memory alloy which exhibits either shape memory or pseudoelastic properties or from an elastic material having an inherent spring tension.
In a first embodiment of the invention, the endoluminal stent is encapsulated in the ePTFE covering in the stent's reduced diametric dimension and is balloon expanded in vivo to radially deform the ePTFE covering. The endoluminal stent may be either one which exhibits thermal strain recovery, pseudoelastic stress-strain behavior or elastic behavior at mammalian body temperature. While in its reduced diametric dimension the ePTFE encapsulating covering integrally constrains the endoluminal stent from exhibiting either thermal strain recovery, pseudoelastic stress-strain behavior or elastic behavior at mammalian body temperature. Radial deformation of the ePTFE covering releases constraining forces acting on the endoluminal stent by the undeformed ePTFE covering and permits the stent to radially expand.
In a second embodiment of the invention, an endoluminal stent fabricated of a shape memory alloy is encapsulated in its final diametric dimension and the encapsulated intraluminal stent-graft is manipulated into its reduced diametric dimension and radially expanded in vivo under the influence of a martensite to austenite transformation.
In a third embodiment of the present invention, a self-expanding intraluminal stent, fabricated of a material having an inherent spring tension, is encapsulated in its final diametric dimension and manipulated to a reduced diametric dimension and externally constrained for intraluminal delivery. Upon release of the external constraint in vivo the spring tension exerted by the self-expanding stent radially expands both the stent and the ePTFE encapsulating covering to a radially enlarged diameter.
In a fourth embodiment of the invention, the endoluminal stent is fabricated from a material having an inherent elastic spring tension and is encapsulated at a reduced dimension suitable for endoluminal delivery and balloon expanded in vivo to radially deform the ePTFE covering.
Shape memory alloys are a group of metallic materials that demonstrate the ability to return to a defined shape or size when subjected to certain thermal or stress conditions. Shape memory alloys are generally capable of being plastically deformed at a relatively low temperature and, upon exposure to a relatively higher temperature, return to the defined shape or size prior to the deformation. Shape memory alloys may be further defined as one that yields a thermoelastic martensite. A shape memory alloy which yields a thermoelastic martensite undergoes a martensitic transformation of a type that permits the alloy to be deformed by a twinning mechanism below the martensitic transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent austenite phase. The austenite phase occurs when the material is at a low strain state and occurs at a given temperature. The martensite phase may be either temperature-induced martensite (TIM) or stress-induced martensite (SIM).
When a shape memory material is stressed at a temperature above the start of martensite formation, denoted Ms, where the austenitic state is initially stable, but below the maximum temperature at which martensite formation can occur, denoted Md, the material first deforms elastically and when a critical stress is reached, it begins to transform by the formation of stress-induced martensite. Depending upon whether the temperature is above or below the start of austenite formation, denoted As, the behavior when the deforming stress is released differs. If the temperature is below As, the stress-induced martensite is stable; however, if the temperature is above As, the martensite is unstable and transforms back to austenite, with the sample returning to its original shape. U.S. Pat. Nos. 5,597,378, 5,067,957 and 4,665,906 disclose devices, including endoluminal stents, which are delivered in the stress-induced martensite phase of shape memory alloy and return to their pre-programmed shape by removal of the stress and transformation from stress-induced martensite to austenite.
Shape memory characteristics may be imparted to a shape memory alloy by heating the metal at a temperature above which the transformation from the martensite phase to the austenite phase is complete, i.e., a temperature above which the austenite phase is stable. The shape of the metal during this heat treatment is the shape “remembered.” The heat treated metal is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform to the martensite phase. The metal in the martensite phase is then plastically deformed, e.g., to facilitate its delivery into a patient's body. Subsequent heating of the deformed martensite phase to a temperature above the martensite to austenite transformation temperature, e.g., body temperature, causes the deformed martensite phase to transform to the austenite phase and during this phase transformation, the metal reverts back to its original shape.
The term “shape memory” is used in the art to describe the property of a material to recover a pre-programmed shape after deformation of a shape memory alloy in its martensitic phase and exposing the alloy to a temperature excursion through its austenite transformation temperature, at which temperature the alloy begins to revert to the austenite phase and recover its preprogrammed shape. The term “pseudoelasticity” is used to describe a property of shape memory alloys where the alloy is stressed at a temperature above the transformation temperature of the alloy and stress-induced martensite is formed above the normal martensite formation temperature. Because it has been formed above its normal temperature, stress-induced martensite reverts immediately to undeformed austenite as soon as the stress is removed, provided the temperature remains above the transformation temperature.
The martensitic transformation that occurs in the shape memory alloys yields a thermoelastic martensite and develops from a high-temperature austenite phase with long-range order. The martensite typically occurs as alternately sheared platelets, which are seen as a herringbone structure when viewed metallographically. The transformation, although a first-order phase change, does not occur at a single temperature but over a range of temperatures that varies with each alloy system. Most of the transformation occurs over a relatively narrow temperature range, although the beginning and end of the transformation during heating or cooling actually extends over a much larger temperature range. The transformation also exhibits hysteresis in that the transformations on heating and on cooling do not overlap. This transformation hysteresis varies with the alloy system.
A thermoelastic martensite phase is characterized by having a low energy state and glissile interfaces, which can be driven by small temperature or stress changes. As a consequence of this, and of the constraint due to the loss of symmetry during transformation, a thermoelastic martensite phase is crystallographically reversible. The herringbone structure of athermal martensite essentially consists of twin-related, self-accommodating variants. The shape change among the variants tends to cause them to eliminate each other. As a result, little macroscopic strain is generated. In the case of stress-induced martensite, or when stressing a self-accommodating structure, the variant that can transform and yield the greatest shape change in the direction of the applied stress is stabilized and becomes dominant in the configuration. This process creates a macroscopic strain, which is recoverable as the crystal structure reverts to austenite during reverse transformation.
The mechanical properties of shape memory alloys vary greatly over the transformation temperature range. Martensite phase alloys may be deformed to several percent strain at quite a low stress, whereas the austenite phase alloy has much higher yield and flow stresses. Upon heating after removing the stress, the martensite phase shape memory alloy will remember its unstrained shape and revert to its austenite phase.
Where a shape memory alloy is exposed to temperature above its transformation temperature, the martensite phase can be stress-induced. Once stress-induced martensite occurs, the alloy immediately strains and exhibits the increasing strain at constant stress behavior. Upon unloading of the strain however, the shape memory alloy reverts to austenite at a lower stress and shape recovery occurs, not upon the application of heat but upon a reduction of stress. This effect, which causes the material to be extremely elastic, is known as pseudoelasticity and the effect is nonlinear.
The present invention preferably utilizes an binary, equiatomic nickel-titanium alloy because of its biocompatibility and because such an alloy exhibits a transformation temperature within the range of physiologically-compatible temperatures. Nickel-titanium alloys exhibit moderate solubility for excess nickel or titanium, as well as most other metallic elements, and also exhibits a ductility comparable to most ordinary alloys. This solubility allows alloying with many of the elements to modify both the mechanical properties and the transformation properties of the system. Excess nickel, in amounts up to about 1%, is the most common alloying addition. Excess nickel strongly depresses the transformation temperature and increases the yield strength of the austenite. Other frequently used elements are iron and chromium (to lower the transformation temperature), and copper (to decrease the hysteresis and lower the deformation stress of the martensite). Because common contaminants such as oxygen and carbon can also shift the transformation temperature and degrade the mechanical properties, it is also desirable to minimize the amount of these elements.
As used in this application, the following terms have the following meanings:
Af Temperature: The temperature at which a shape memory alloy finishes transforming to Austenite upon heating.
As Temperature: The temperature at which a shape memory alloy starts transforming to Austenite upon heating.
Austenite: The stronger, higher temperature phase present in NiTi.
Hysteresis: The temperature difference between a phase transformation upon heating and cooling. In NiTi alloys, it is generally measured as the difference between Ap and Mp.
Mf Temperature: The temperature at which a shape memory alloy finishes transforming to Martensite upon cooling.
Ms Temperature: The temperature at which a shape memory alloy starts transforming to Martensite upon cooling.
Martensite: The more deformable, lower temperature phase present in NiTi.
Phase Transformation: The change from one alloy phase to another with a change in temperature, pressure, stress, chemistry, and/or time.
Shape Memory: The ability of certain alloys to return to a predetermined shape upon heating via a phase transformation.
Pseudoelasticity: The reversible non-linear elastic deformation obtained when austenitic shape memory alloys are strained at a temperature above As, but below Md, the maximum temperature at which pseudoelasticity is obtained.
Thermoelastic Martensitic Transformation: A diffuisionless, thermally reversible phase transformation characterized by a crystal lattice distortion.